They would see red flags and/or press me/probe me for more details and really assess whether I am an imminent risk or danger to myself. If so, then it's likely they would take measures to intervene. I would of course, be resentful if I had truly planned to die (not impulsively or anything, but well thought out) and someone got in the way and interfered with that. I do know that if somehow I was stopped before doing it, then I'd likely try again (probably with a more risky method) partly out of spite towards society and the people who wouldn't respect my wishes or choice. Since I have no immediate plan on dying anytime soon, I'm just coping with life on a daily basis.
Also, others have good responses, especially @Futility about the hypocrisy of humans towards each other (they would rather a fellow human being suffer while letting an animal go with dignity and as peacefully as possible).